Distributed generation (DG) is the use of power generation technologies located close to one or more loads being served. Distributed energy resources (DER) is another term describing the use of one or more DG sources and/or other components that are interconnected with the utility grid. Such interconnection involves assembling the various components in such a manner that they satisfy the requirements of the utility service provider that operates the utility grid in the jurisdiction. These requirements include electrical equipment safety standards and regulations promulgated by the Institute of Electrical and Electronic Engineers (IEEE) and the Underwriters Laboratory (UL). For example, interconnection requirements for DER systems are provided in the “Point of Common Coupling” (PCC) standard of IEEE 1547. As another example, the UL requires each DG source that interconnects with the utility grid to individually satisfy the requirements of UL 1741.
Distributed power generation is becoming more common throughout the world as alternative energy sources are being used to generate electric power. In the United States, deregulation of electric utilities has influenced the development of independent energy sources, which are tied into the utility grid. Typical DG sources include turbine generators, internal combustion engines/generators, micro-turbines, gas turbines, photovoltaics/solar panels, wind turbines, fuel cells, batteries, capacitor-type storage systems, and flywheel energy storage systems. DG stakeholders include energy companies, equipment suppliers, regulators, energy users and financial and supporting companies.
Examples of distributed power generation include a hospital operating a reciprocating engine for standby power or to enable peak shaving, a computer chip manufacturer operating a fuel cell and using DG sources to ensure suitable power quality, a chemical plant operating a combustion turbine, and residential sources such as photovoltaics, batteries, and fuel cells. Common DG applications include load reduction, standby power, peak shaving, net metering, residential solar, combined with heat (CHP), grid support, premium power, island systems, and remote agricultural. Common configurations include grid connected, grid independent, hybrid systems, and mini-grid.
DG systems can provide a number of benefits, depending upon the particular system, including but not limited to: greater reliability, customized solutions, premium power capability, savings when combined with heat (CHP), reduced demand charges, power in remote and islanded conditions, environmental benefit if renewable resources are used, and energy independence.
However, current DG systems have various limitations, for example, small grids generally do not have much load or generation averaging, and need more voltage and frequency stabilization, on the order of 10-20%, as compared to most utility grids which require 1-2% adjustment for voltage and frequency stabilization. Also, DG sources tend not to follow loads well. Fuel cells, wind, micro-turbines, and natural gas sources are reciprocating, but require significant VARs and/or responsive energy storage. Other technical limitations include the lack of a single standard for communicating with DG systems, the need for back-up, black start, and overloads, and conflicts between anti-islanding schemes.
Interconnection issues affect both DG users and utilities. The DG user would like to receive voltage support, frequency regulation, and back-up from the utility grid, but often is subjected to substantial charges just to receive “free” VARs. Moreover, control standards for hybrid systems can be difficult for the DG user to follow. On the other hand, utilities receive a high demand for VARs, which are generated at the expense of real power sales. Utilities also are concerned about islanding and safety issues, the variability of net power, communication, setting rates, and not receiving congestion relief if back-up is required. Although DG users require a connection to the grid, utility operators see DG as a burden.
Commercial utility customers and large residential customers that support significant loads usually maintain a primary connection to the utility grid, but sometimes utilize back-up power generation sources to service their loads in the event of a power disruption. Such customers can be connected to the utility grid in one of two basic arrangements: a traditional one-way arrangement and a two-way arrangement. In the traditional one-way arrangement, the utility customer operates back-up power generation sources such as batteries or diesel generators only in the event of a power disruption that interferes with the supply of power from the utility grid. Otherwise, the back-up sources generally are not used.
In a two-way arrangement, the power generation sources are arranged to provide distributed generation (DG) and operate on a periodic or substantially continuous basis. In two-way arrangements, one or more loads and the DG sources are connected to form a mini-grid having a connection to the utility grid. A mini-grid typically refers to one or more independent energy sources (e.g., DG sources), one or more loads, and other components such as electrical connection circuitry that are linked together, where the mini-grid usually maintains a primary connection to the utility grid.
With two-way arrangements, the utility customer remains connected to the utility grid, and becomes a net purchaser of power during peak load operating conditions; for example, a factory would buy power during hours of peak demand. The utility customer then sells back unused power generated by the DG sources during off-peak hours or at times when load demands are minimal.
Conventionally, to satisfy peak-hour load demands, it is necessary and desirable for operators of mini-grids that include DG sources to maintain a connection to the utility grid. Without such a continuous connection, the mini-grid can suffer from power quality and reliability problems. These problems revolve around two major system parameters: frequency stability and voltage regulation.
Frequency and voltage are related to the two types of power supplied by the utility service provider to the utility grid: real power and reactive power. Real power is the power that performs real work, and is equal to the time average of the instantaneous product of voltage and current. Reactive power is the power that exists in the electrical distribution and transmission system due to the inherent capacitive and inductive elements in the system. Reactive power, also known as imaginary power, is also sometimes referred to as “VAR” or “VARs”. Reactive power is the instantaneous product of voltage and current, where the current is phase shifted plus or minus 90 degrees.
Both real power and reactive power are generated and supplied to utility customers over the utility grid. Real power generation is associated with frequency stability. In the United States, electric current is generated so as to provide an alternating current (AC) at a desired frequency of 60 Hz. In order to keep the AC frequency within desired limits and/or ranges, the difference between the load and the power being generated is determined periodically, and this difference is used to increase or decrease the output of generators/power generation facilities. Typically, about one percent (1%) of the power being transmitted and/or capable of being generated and delivered to the utility grid is reserved to fine tune or regulate the AC frequency of the electrical power being distributed. A system for regulating the frequency of generated power using flywheel energy storage systems is described in U.S. Ser. No. 10/920,146, filed on Aug. 16, 2004, which is incorporated by reference herein. Utility customers rely on a continuous connection with the utility grid to provide frequency stability, which prevents damage to electrical equipment and loads.
Although utility customers are directly billed (e.g., in kilowatt-hours) for the real power consumed by their loads linked to the utility grid, access to reactive power (or VARs) is equally important. Reactive power supplied through the utility grid is critical for providing voltage regulation or load matching capabilities. In conventional mini-grid arrangements, there is a hazard to load equipment which can be damaged by unstable grid voltage or frequency whenever the mini-grid is disconnected from the utility grid. Continuous monitoring and control of the mini-grid must be provided by remote and local operators, since DER systems are sensitive to electric power disturbances from the utility grid, and vice versa.
When a mini-grid is disconnected from the utility grid, DG sources of the mini-grid become tied directly to one or more loads. In cases where there is a disabled connection between the mini-grid and the utility grid, a condition known as “islanding” occurs. Islanding is defined for purposes of this application as the continued operation of a DER system or mini-grid when the utility grid has been switched off or otherwise disconnected from the mini-grid so that no electric energy flows from the utility grid to the mini-grid.
Conventionally, in an island condition, the DG sources of the mini-grid are directly linked to the one or more loads, and no real power or reactive power is delivered by the utility grid. In some cases, the mini-grid remains connected to the utility grid so that electric power is supplied from the mini-grid to the utility grid, which can result in quality problems for the utility service provider because the power supplied to the utility grid may not satisfy voltage and frequency requirements. An anti-islanding power converter has been developed to protect against the problems associated with islanding conditions. As described in U.S. Pat. Nos. 6,219,623 and 6,810,339, such converters are used to isolate DG sources from the utility grid when voltage or frequency measurements are outside acceptable ranges.
When the mini-grid operates in an islanding condition, such that no electric energy flows from the utility grid to the mini-grid, the DG sources of the mini-grid may not supply sufficient real power or reactive power to satisfy the requirements of the load-bearing equipment. Even if the real power generated temporarily matches the loads, there may be problems associated with frequency regulation and voltage regulation, as discussed above. Moreover, the selection or combination of DG sources linked to the mini-grid may result in load-following or power quality problems. For example, fuel cells may trip offline due to large transient cyclic loads, while synchronous generators tend to simply shut down when the mini-grid is isolated or disconnected from the utility grid.
DER systems capable of being monitored and controlled remotely are desirable for utility and area operators. Unfortunately, the lack of standardization in communication protocols and physical connections for DER systems often makes it impractical for utilities and area operators to gain access to these mini-grids/DER systems. In most instances, utilities only require aggregated data such as total production, total available capacity, and operational state from the DER systems. The situations is further complicated by sites with multiple DER systems and mini-grids requiring separate communication channels.
It thus would be desirable to provide a universal device, system, and method for connecting mini-grids operating under various communication protocols with the utility grid.
It also would be desirable to provide systems and methods for allowing continued operation of DG sources in a mini-grid to supply the one or more loads connected to the mini-grid upon disconnection from the utility grid. In other words, it would be desirable to intentionally isolate mini-grids of DER systems/DG sources in the event of disruption or disconnection from the utility grid. Presently available devices and techniques do not provide substantial frequency stability and load-following capabilities to permit substantially continuous operation of a mini-grid when disconnected from the utility grid.